Mode-locked lasers can provide accurate frequency combs. An optical frequency comb is an optical spectrum of equidistant lines. The use of frequency combs as a tool may depend on the spacing between teeth of the comb being sufficiently large for a given application. A proposed method to increase tooth spacing has included the use of a Fabry-Perot cavity to multiplex the pulse train of a mode-locked laser. The shortcomings of this method are threefold. First, because the device is a passive device, the average power is reduced. Second, because of the unavoidable cavity dispersion, the unequal spacing of the Fabry-Perot modes defeat the purpose of a comb of absolute accuracy. Even when the cavity is in vacuum, there will be dispersion due to the minor changes. Finally, matching the carrier-to-envelope offset (CEO) of the driving laser and slave cavity is an unsolved problem. The CEO is an important parameter of pulse trains related to the change in phase from pulse to pulse in the train.